var guessNumber = function(n) {
    let left = 1, right = n;
    while (left < right) { // 循环直至区间左右端点相同
        const mid = Math.floor(left + (right - left) / 2); 
        if (guess(mid) <= 0) {
            right = mid; // 答案在区间 [left, mid] 中
        } else {
            left = mid + 1; // 答案在区间 [mid+1, right] 中
        }
    }
    // 此时有 left == right，区间缩为一个点，即为答案
    return left;
};
